1. Field of the Invention
The present invention relates to a converter control circuit and a converter having the same, and more particularly to a converter control circuit which is capable of changing an operation mode, and a half-bridge converter having the same.
2. Description of the Related Art
Recently, with the advance of electronic appliances, integrated circuit (IC) technologies have been utilized in these appliances to improve their performance while making their size and weight small. Further, the smallness and lightness of power supplies indispensable to such electronic appliances have also been required. Switched mode power supplies are mainly used as stabilized power supplies meeting such requirements. A direct current (DC)-DC converter, which is one of the switched mode power supplies, is a device for converting a given DC voltage into a voltage of a desired DC level. The DC-DC converter performs the voltage conversion by first converting a DC voltage into an alternating current (AC) voltage through a switching operation, and then stepping the converted AC voltage up or down through a transformer and rectifying the stepped-up or down AC voltage. This converter may be of various types including a forward type, flyback type, half-bridge type, and full-bridge type. An asymmetric pulse width modulation (PWM) half-bridge converter, resonant half-bridge converter, etc., derived from the half-bridge type among the various types, can reduce switching losses through zero voltage switching, so that they are widely used in applications requiring high efficiency.
FIG. 1 is a circuit diagram showing the configuration of a conventional half-bridge DC-DC converter.
The conventional half-bridge DC-DC converter comprises a DC voltage source Vin for supplying a DC voltage, a first switching device S1 and second switching device S2 connected between both ends of the DC voltage source Vin, a first capacitor C1 connected to a connection point of the DC voltage source Vin and the second switching device S2, a transformer T having a primary winding L1 connected between the first terminal of the first capacitor C1 and a connection point of the first and second switching devices S1 and S2, a first rectifier 110 and second rectifier 120 connected to a first secondary winding L2 and second secondary winding L3 of the transformer T, respectively, a smoothing circuit 130 connected between output terminals of the first rectifier 110 and second rectifier 120 and a connection point of the first secondary winding L2 and second secondary winding L3, and a switching controller 140 for receiving a secondary voltage of the transformer T fed back thereto and controlling a switching operation of the first switching device S1 and the second switching device S2 based on the received secondary voltage. The first rectifier 110 and second rectifier 120 include a diode D1 and diode D2, respectively, and the smoothing circuit 130 includes an inductor L4 and a second capacitor C2.
FIGS. 2A and 2B are timing diagrams illustrating the operations of the first switching device S1 and second switching device S2 of the converter shown in FIG. 1: FIG. 2A(A) and FIG. 2B(A) are timing diagrams of the first switching device S1, and FIG. 2A(B) and FIG. 2B(B) are timing diagrams of the second switching device S2.
The first switching device S1 is turned on for a predetermined time interval t1 by a switching control signal outputted from the switching controller 140. At the primary side of the transformer T a DC input voltage is generated from the DC voltage source Vin and a current loop through the first capacitor C1, the primary winding L1 of the transformer T, and the first switching device S1. As a result, the first capacitor C1 is charged. During this time interval t1, due to a voltage induced from the primary side of the transformer T, a current loop is formed at the secondary side of the transformer T through the first secondary winding L2 of the transformer T, the first diode D1, the inductor L4, and the second capacitor C2. Consequently, a DC output voltage Vo is supplied to a load (not shown).
Thereafter, the first switching device S1 and the second switching device S2 are turned off for a predetermined time interval t2 by the switching control signal from the switching controller 140. This time interval t2 is called a dead time t2. The dead time t2 is provided to prevent the input voltage Vin from being grounded or shorted, by inhibiting the second switching device S2 from being turned on at the same time that the first switching device S1 is turned off.
After the dead time t2, the second switching device S2 is turned on for a predetermined time interval t3 by the switching control signal outputted by the switching controller 140. As a result, the first capacitor C1 is discharged through a current loop through the second switching device S2 and the primary winding L1 of the transformer T. During this time interval, due to a voltage induced from the primary side of the transformer T, a current loop is formed at the secondary side of the transformer T through the second secondary winding L3 of the transformer T, the second diode D2, the inductor L4, and the second capacitor C2. Consequently, a DC output voltage Vo is supplied to a load (not shown).
The first switching device S1 and the second switching device S2 are controlled by the switching controller 140 in such a manner that they are alternately turned on for periods T1 and T2, excluding the dead time interval t2.
On the other hand, the output voltage of the converter is generally controlled in a duty ratio control mode (case of FIG. 2A) or frequency control mode (case of FIG. 2B). In the duty ratio control mode, the output voltage is controlled based on a duty cycle under the condition that a frequency is fixed. In the frequency control mode, the output voltage is controlled based on a switching frequency under the condition that a duty ratio is fixed to 50%. The frequency control mode is used in a resonant half-bridge converter, among various converters, whereas the duty ratio control mode is used in an asymmetric PWM half-bridge converter and an active clamp converter.
Because the asymmetric PWM half-bridge converter and the resonant half-bridge converter use the different control modes as stated above, they employ different control circuits. For this reason, where a system uses different converters, it must have different control circuits based on the used converters, resulting in the occurrence of various problems including increases in the size and manufacturing cost of the system. Therefore, there is a need for a multipurpose converter control circuit applicable to various topologies including the asymmetric PWM half-bridge converter, active clamp converter and resonant half-bridge converter.